ventilation verification for gas tank

[64_twins]

Thanks. Just to make sure I am clear on the best way to avoid problems. There are three additional ports on my tank besides the filler. I hooked a hose from my filler neck to one of the ports as I thought that might be for overflow. On the other two. Should I put a check valve on each one or just one of them? If only one should I cap the other off or put a different type valve on it for when I'm adding fuel?